Angle modulation is the generic term for two closely related types of modulation of an electric wave, namely phase modulation and frequency modulation.
Phase modulation is a modulation method in which the phase angle of a base or carrier wave is varied depending on a parameter, such as the amplitude, of a modulation signal which represents the information to be transmitted. The difference of the instantaneous phase from a reference phase is a function of the instantaneous value of the modulation signal. Frequency modulation is a modulation method in which the difference between the instantaneous value of the frequency of a carrier wave and a reference frequency is a function of the instantaneous value of the modulation signal.
In a known phase modulation method
a. two carrier frequency waves having phases differing by a predetermined angle, e.g. 90.degree. are generated, PA0 b. the amplitude of at least one of these waves is modulated by a modulation signal, PA0 c. the waves are then added to form a combined wave.
The amplitude of the combined wave may be limited to suppress amplitude variations.
In a known modification of the above method, both waves are amplitude modulated in push-pull relationship by the modulation signal.
U.S. Pat. No. 3,887,786, incorporated herein by reference thereto, discloses a quadrature amplitude modulation (QAM) method in which the signal values or points, as mapped on the complex plane, are drawn from an alphabet consisting of at least eight points and are set up in concentric rings each rotated by 45.degree. with respect to adjacent rings. respectively, of the signal point to be transmitted. The amplitude modulated pulse trains are low pass filtered and modulate waves of the form cos w.sub.c t and sin w.sub.c t, respectively, wherein w.sub.c is a carrier frequency. The modulated cosine wave is applied to the non-inverting input of a differential amplifier while the modulated sine wave is applied to the inverting input of the differential amplifier which produces the desired QAM modulated signal at its output.
A modulating signal representing digital information can assume only a number of discrete values which in the case of phase modulation differ by a predetermined incremental phase angle, such as 45.degree.. This causes abrupt phase variations or phase jumps in the phase-modulated signal. An analog modulating signal may also cause abrupt phase variations if the signal parameter, e.g. the signal amplitude, which represents the information to be transmitted, changes aabruptly, as in television signals.